Using a 3G Antenna to improve reception for your 3G Router
Its critical to consider the distance from the transmission tower and any obstacles that lie between the antenna and the tower. These factors also affect outdoor antennas, but it is more critical to pay attention to these details since indoor antennas are usually having top cope with trying to receive the signal through the walls of your property which will obviously have a considerable effect on the signal strength and signal quality (the amount of noise on the signal).
Distance From Transmission Tower
There isnt a specific distance that determines if an indoor antenna will work for you. If you live within a built up area then you will likely be able to use an indoor antenna. If you are in a rural area where the mobile phone cell density is lower then you might have to resort to an outdoor antenna (which is generally higher gain and can be mounted in an outdoor position which gives better clear line of site to the nearest transmission mast).
Obstacles Between Antenna and Transmission Tower
Obstacles can be mountains, hills, buildings, walls, doors, people walking in front of the antenna, etc. These create havoc with all wireless signals and impact the reliability in signal reception. The effects of obstacles in the signal path are impossible to predict. You might notice that if you configure the antenna direction just right, get the right atmospheric conditions and open the front door or blinds then it comes in without issue. The door and blinds can be considered obstacles to your reception. Also, reception cuts in and out when someone walks in front of the antenna. These are all factors which can effect indoor reception. Therefore, when comparing indoor to outdoor antennas, indoor antennas typically have a shorter reception range, easier to install and cost less BUT the outdoor antenna will generally allow much better signal reception.
Directional vs. Omni Antenna
The first thing to note is that an antenna does not BOOST a signal. An antenna simply concentrates a signal i.e. it takes signal from an unused direction and concentrates it into a useful direction. Generally there are two main types of antenna: Directional and Omni. A directional antenna typically concentrates the signal into a cone which you aim at the signal source. If you have a clear line of site between your receiver and the transmission tower then a directional antenna is generally the best type. An omni antenna concentrates the signal into a horizontal disc: Its taken signal that would normally be directed upward or downward and sent into a thin disc. An omni antenna is generally your best choice where you are unsure where the signal is coming from or the signal is being received after reflection or scatter due to obstructions in the way.
dBi is the measuring unit for the gain of an antenna. The reference level or dBi is the strength of the signal that would be transmitted by a non-directional isotropic antenna i.e. radiates equally in all directions. This antenna exists as a mathematical concept used only as a known reference to measure antenna gain per dBi. In electronics, the term "gain" is often repeated but misunderstood. Gain implies increase e.g 20 dBi but without respect to where the increase originated.
An antenna transmits and receives radio waves. Antenna gain is used to indicate the increase in power of one antenna (transmitting or receiving) as compared to another antenna. Gain is actually a ratio of power levels and is stated in decibels dBi. The dipole or basic antenna concentrates it signals in two directions. The isotropic antenna doesn't favour any particular direction so its dBi gain equals 0. The dipole has a 2.1 dBi measurement gain over an isotropic radiator. Therefore a 6 dBi antenna gain over an isotropic radiator computes to a 3.9 dBi gain over a dipole. For every 3 dBi of improvement added to your antenna, results in a noticeable effect on the receiving station. Any dBi gain less than 3 dBi leads to an undetectable dBi improvement, however don't discount improvements under 3 dBi. Sometimes even a antenna gain of .5 dBi can make the difference between receiving and not receiving a wireless signal!
The important point to remember though is the extra gain of an antenna has been achieved by concentrating the signal and hence, narrowing the useable reception beamwidth. Typically, if you double the power (gain) of an antenna then youve halved the area of beam coverage. This is an important consideration when you consider how easy or difficult its going to be to aim the antenna at the signal source: Obviously it will be easier with a low gain antenna with a wider beamwidth than it will with a much higher gain with a much tighter beam angle. Deciding on which antenna gain to use is often a compromise between signal strength and signal coverage.